This invention relates in general to submersible pumps and in particular to an electrical circuit for preventing restarting of the pump motor while the pump is backspinning.
Submersible pumps are often used in deep wells for pumping large volumes of liquid to the surface. Often, the pump assembly will be located several thousand feet into the well. The pump assembly normally includes a centrifugal pump, below which is mounted a large alternating current electrical motor for driving the pump.
For various reasons, pumps are often automatically shut down. This could be due to momentary overload or power fluctuations. When the power to the electrical motor is cut off, the motor will continue to spin along with the pump in a forward direction for a period of time due to the momentum. Then, the motor will cease spinning in a forward direction and start backspinning. The spinning in the reverse direction is due to the column of liquid above the pump falling downward into the well. Depending on the well, several thousand feet of liquid above the pump may drop past the pump.
It is important that while the motor is backspinning at a fairly high rate that no attempt be made to start the motor. Some circuits have automatic starting devices that may attempt restarting without being aware of backspinning. Also, field personnel may be present that might attempt to manually start the motor while the motor is backspinning. The sudden surge of power to the motor for starting while it is backspinning creates extremely high torque on the shaft and may cause the motor shaft to twist in two.